This invention relates generally to electrooptical devices and, more specifically, to a novel technique for providing an electrical bias in such devices. In recent years, devices have been developed to make use of an electrooptical effect in which an electric field induces a change in refractive index of an electrooptic crystal. An optical field propagating in a waveguide formed in the crystal is phase modulated by the induced change in refractive index. The electrooptical effect can be used in devices such as modulators and switches.
Most electrooptical devices using phase modulation require some kind of biasing structure to produce an electric field that compensates for differences in propagation constants and other differences arising from difficulties with manufacturing tolerances. Biasing is also needed to bring the device to a desired operating point in its phase modulation characteristic. The use of biasing for phase tuning allows the device to achieve low cross-talk, in the case of electrooptical switches, and low harmonic distortion, in the case of electrooptical modulators.
Prior to this invention, phase tuning involved loading the waveguides with dielectric or metal films. Precise phase tuning by this technique is very difficult, since the resultant phase shifts cannot be precisely determined in advance of fabrication and cannot be subsequently tuned. A more reliable technique is to supply a biasing field with an additional pair of electrodes and an external voltage source. This approach increases the circuit complexity, and the requirement for an external voltage source is usually a significant inconvenience.
Accordingly, there is still a need for another, more convenient technique for supplying a biasing electric field in an electrooptical device. The present invention satisfies this need.